Plugin: handover step + auto-seed prior notes into next session (hc-dy9)

Add a final "Notes for next time" step so the tech finishing today can flag
pending issues, watch-fors, and outstanding client decisions for whoever
picks up the next healthcheck on the same site.

On ATT_HC_Session::start() for a given site_key, the server's step history
for the handover step is queried (limit 1, excluding the just-created
session). If a prior session left handover notes, they're written into
the new session's "Before You Start" notes prefixed with the prior
session's date ("From previous session (YYYY-MM-DD):") so the carry-over
is obvious. The tech can edit/clear them as normal step notes from there.

- includes/steps/125-handover.php — new step (id=handover) using the
  standard notes field. No server schema or API change; it's just another
  step row in step_updates, surfaced like any other.
- ATT_HC_Session::seed_before_notes_from_prior_handover() — best-effort,
  silent degrade on API failure. The session is already registered on
  the server before this runs, so a failed seed never blocks start.
- No seed on resume() — resuming an existing session would clobber
  whatever the tech had already typed.

Verified end-to-end against the live MySQL server: handover-test-XXXX
flow shows carry-over with date prefix; no-handover-XXXX flow confirms
no false-positive seed for a fresh site_key. Test rows purged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

includes/class-att-hc-session.php

@@ -84,7 +84,39 @@ final class ATT_HC_Session {
         ]);
 
         update_option(ATT_HC_OPT_SESSION, $data, false);
-        return new self($data);
+        $session = new self($data);
+
+        // Carry handover notes from the prior session into the new "before"
+        // notes. Best-effort: a failure here must not block session creation,
+        // since the session is already registered on the server above.
+        self::seed_before_notes_from_prior_handover($session);
+
+        return $session;
+    }
+
+    /**
+     * If the prior session for this site left "Notes for next time" (id=handover),
+     * pre-fill the new session's "Before You Start" (id=before) notes with them,
+     * prefixed with the prior session's date so it's clear they're carried over.
+     */
+    private static function seed_before_notes_from_prior_handover(self $session): void {
+        try {
+            $resp = ATT_HC_Api::step_history('handover', $session->site_key(), 1, $session->id());
+        } catch (ATT_HC_Api_Exception $e) {
+            return;
+        }
+        $rows = $resp['history'] ?? [];
+        if (!$rows) return;
+
+        $notes = trim((string) ($rows[0]['notes'] ?? ''));
+        if ($notes === '') return;
+
+        $prefix = 'From previous session (' . date('Y-m-d', (int) $rows[0]['started_at']) . "):\n";
+        try {
+            $session->update_step('before', self::STATUS_NOT_STARTED, $prefix . $notes);
+        } catch (ATT_HC_Api_Exception $e) {
+            // Already-rare degrade path — the session is alive, just no auto-seed.
+        }
     }
 
     /**